As an important parameter, start current reveals the excitation condition of cavity modes. Normally, the method of successive iterations with one mode associated in the calculation is used to get the start current, which just reflects the self-excitation conditions in the presence of single mode. Nevertheless, in many cases, the prior-excited mode may affect the start current of the subsequent ones. In this article, we re-derived electron motion equations with two modes involved in the calculation and calculated the start current numerically based on those equations. We analyzed start currents of operation mode TE26+ and parasitic modes of a 394-GHz gyrotron with the two methods. In the calculation using the conventional method, TE26+ satisfies self-excitation condition while TE26− does not. In the calculation using the latter method, the start current of TE26− decreases under the influence of TE26+. This nonlinear oscillation of TE26− is further observed in time domain multimode simulation. In practical application, the beam velocity spread, ohmic loss, and high-frequency loss are inevitable. We studied their effects on mode interaction and then optimized the operation parameters. When the equivalent conductivity is $1.7e^{7}~\text {s/m}$ and beam velocity spread is 0.03, the optimized output power is 149.2 W.

The Ka-band extended interaction klystrons (EIK) have been developed at the Institute of Electronics, Chinese Academy of Sciences (IECAS), to satisfy the requirement for the high-power millimeter-wave sources in scientific researches. In this work, two kinds of high compression electron gun with the solid or hollow beams, the derivative electron optics systems using the compact permanent magnet uniform focusing and the shared efficient interaction circuit based on the multigap ladder cavity, have been studied in detail and well designed through a mass of calculations. After the mechanical design, finely manufacturing, and a series of technical processes, the Ka-band EIK prototype tubes have been successfully built and tested. For the solid beam scheme, the maximum output power achieves 20 kW in the bandwidth of 60 MHz, and, correspondingly, the gain of 53 dB and the efficiency of 24% are observed. The measured -1 and -3 dB bandwidths are about 220 and 350 MHz, respectively. Meanwhile, the beam transmission is excellent, that is over 97% in dc state and over 90% in RF operation. For the hollow beam case, an over 91% dc transmission test has finished.

After completing the designs of the electron gun with large compression ratio, the stable electron optics system with high transmission, and the efficient beam-wave interaction circuit, using a permanent magnetic uniform field focusing, we have successfully built a high power Ka-band extended interaction klystron with excellent performance. The working voltage and current are about 25.5 kV and 3.1 A, respectively. The measured DC beam transmission is over 97%, and in the most adverse state of high frequency operation, the transportation can still keep the value of over 90%. In the condition of non-equal excitation, the device demonstrates the bandwidth of 435 MHz corresponding to the output power of 10 kW or more. The duty cycle is over 3%. Near the optimal working point, the klystron clearly shows its potential, which means that the maximum output power exceeds 20 kW with the gain 44 dB and the efficiency 25% in the bandwidth of 60 MHz.

推导出了考虑损耗层厚度影响的有限电导率材料加载同轴波导中横电模传输常数的解析表达式.结合同轴波导回旋行波管的非线性理论对分布损耗涂层加载的W波段TE02模同轴波导回旋行波管的互作用系统进行了稳定性研究,分析了互作用系统中的不稳定问题.研究表明:互作用电路各区(线性、非线性区)长度的变化不但会导致互作用系统最危险竞争模式的变化,还会导致竞争模式的局部轴向模发生转移,合理选择互作用段的长度是稳定工作模式的重要条件;相比普通圆波导结构,采用同轴波导结构,稀释了工作模式附近的模式密度,并通过分布损耗加载,提高了竟争模式起振电流,相应地提高了工作模式的稳定性.基于稳定性分析结果,在电压60 kV、电流13 A、电子注速度零散为3％的情况下,W波段同轴波导回旋行波管放大器可获得峰值功率232 kW,-3 dB带宽2.5 GHz的放大性能.

As the magnetic field used in nuclear magnetic resonance (NMR) research increases (up to 23 T), the irradiation frequency of the electromagnetic field used for dynamic nuclear polarization (DNP) needs to be in the terahertz band (140–600 GHz). This article analyzed and compared the influence of different external magnetic field profiles on start current and beam–wave interaction of a 394-GHz gyrotron. As the simulation results show, the minimum start current corresponding to the linear and parabolic external magnetic field profile is lower than that corresponding to the uniform magnetic field. The minimum start current in the case of parabolic magnetic field profile is a little lower than that in the case of a linear magnetic field profile. Based on the nonlinear self-consistent calculation, it is observed that the optimum output powers in both linear and parabolic magnetic field profile cases are nearly equal when the beam current is 0.25 A and both of them are larger than the optimum output power when the magnetic field is uniform. In the calculation of orbital efficiency $\eta _{\bot } $ , the maximum orbital efficiency in the soft-excitation region in the inhomogeneous magnetic field profile cases is larger than that in the case of uniform magnetic field profile. After comprehensive consideration of beam voltage, beam current, and efficiency, the final operation point is located in the place where the beam current is 0.25 A and the beam voltage is 15 kV. The output power is more than 300 W and meets the requirements of DNP-NMR experiment.

This paper derived time domain multimode formula and combined it with the frequency single mode theory to investigate mode oscillation and mode interaction of a 394 GHz gyrotron. The gyrotron reaches a steady state where both TE261+2 and TE261-2 modes oscillate at 393.87 GHz in time domain theory while TE261-2 cannot oscillate in frequency domain theory when the beam voltage is 15 kV, the magnetic field is 7.185 T, and beam current is 0.25 A. TE261+2 mode dominates the final oscillation, which output power and efficiency are 136.8 W and 3.6%, respectively. The output characteristics of the operating mode obtained by two theories are identical.

Relative to the single-cavity gyrotrons operating in high-order mode,the coaxial cavity gyrotrons have the advantages of reducing mode competition,improving the stability of single mode operation and increasing the power capacity. Therefore,the coaxial cavity gyrotrons are more suitable for electron cyclotron resonance heating and electron cyclotron current driving in controlled thermonuclear fusion and attract much attention. The effects of structure parameters,electron beam parameters and ohmic losses on the beam-wave interaction of a coaxial cavity gyrotron operating at 170 GHz, TE(34,11)mode were investigated in detail. Firstly,the mode selection of 170-GHz MW-class gyrotrons was analyzed and mode TE34,11 was chosen as the operating mode. Secondly,based on the time-dependent self-consistent nonlinear theory,a time-domain single-mode steady-state code was written to study the beam-wave interaction. The influences of the beam current,the magnetic field and the ohmic losses on the cavity walls were analyzed and the operating parameters were optimized. The simulation results show that when the voltage,the beam current and the axial guiding magnetic field are designed to be 65 kV,68 A and 6.58 T,an output with 2.18 MW power and 49.23 % efficiency can be obtained,the peak ohmic loss density on the outer wall of the cavity is 1.94 kW/cm(2),and the peak ohmic loss density on the insert is less than 0.15 W/cm(2). The interaction efficiency decreases with the increase of electron velocity spread,and the output frequency shifts downward. The thickness of electron beam has similar effects on the interaction.

In this paper, a design of quasi-optical mode converter for 170-GHz TE32,9-mode high-power gyrotron is presented in detail. The quasi-optical mode converter consists of a launcher and a mirror system. A MATLAB code for analysis of the electromagnetic fields at the launcher wall and in mirror systems has been developed. The numerically optimized launcher can generate a Gaussian-like mode with a scalar correlation coefficient of 99.6% and a vector correlation coefficient of 99.1% at the launcher aperture. The radiation fields of the launcher are analyzed in the free space. The mirror system consists of four analytic surface mirrors and a phase-correction mirror. After the phase correction, the scalar Gaussian mode content and vector Gaussian mode content on the output window are 99.1% and 97.1%, respectively. The performance of the mirrors is calculated and analyzed. The power transmission efficiency of each mirror is above 99.8%, and the efficiency of the quasi-optical mode converter is 96.77%.

Medium-power gyrotron operating in high frequency usually operates in harmonics to reduce the required guiding magnetic field strength. Low guiding magnetic field is in favor of compact gyrotron, which makes it easier to apply the gyrotron to large scale devices and to extend the applications of gyrotron to wider fields. However, high harmonic modes suffer from low intensity of cyclotron radiation and additional competition with neighboring fundamental modes. As we all know, single mode operation can be realized by using a special startup scenario and a positive magnetic field taper can increase output efficiency. The influence of magnetic field taper on gyrotron operation is studied by generalizing the well-known η⊥μ,F plot, which just discussed the transverse efficiency using Gaussian field profile and not took the self-consistent non-stationary codes or ohmic dissipation into consideration. In this paper, linear and self-consistent nonlinear codes are used to investigate influences of magnetic field taper on start current and output parameters of a 394 GHz gyrotron, which is designed for enhancement of Nuclear Magnetic Resonance spectroscopy through Dynamic Nuclear Polarization. Simulation results show that when the magnetic field taper is +0.04 T the maximum output power is obtained and the maximum value is 3% larger than that corresponding to uniform magnetic field. Appropriate startup scenario is designed for single mode operation and multimode simulation is carried out to demonstrate that mode TE261+ dose dominate in mode competition in the designed startup path.

In order to improve the efficiency of the quasi-optical mode converter, two methods to design mirror systems for a 170 GHz gyrotron operating in TE32,9 mode are presented in this paper. The first method is to use Katsenelenbaum-Semenov Algorithm (KSA) to design the structure of the mirror. The second method to design the mirror system depends on the phase difference on the mirrors, so we name it PD method. The mirror system consists of three mirrors, and the mirror center position and mirror size are the same for both methods. For the first method, the scalar and vector correlation coefficients obtained at the window are 99.45% and 98.12%, respectively, and the mirror system has been designed with a transmission efficiency of 97.25%. The scalar and vector correlation coefficients and mirror system transmission efficiency are 99.73%, 98.85%, and 97.67% respectively for the second method. Simulation results of the two methods are compared and analyzed, which provide a reference for the design of gyrotron quasi-optical mode converter mirror system.

In this paper, a W-band rotating TE62 mode is obtained by means of a quasi-optical mode generator in order to test quasi-optical system of the gyrotrons. The quasi-optical generator consists of two mirrors and a coaxial cavity with a perforated outer wall. The simulation results with an electromagnetic analyses software show that the mode purity is up to 96.5%. Processing technologies for components of the mode generator are finished. The assembly device is finished on the basis of the multidimensional localization, which has been tested on the multidimensional automatic test platform.

The design and experiment of a pulsed Ka-band TE01-mode gyrotron traveling-wave tube are presented. This tube was designed to use the fundamental cyclotron resonance interaction with TE01 cylinder waveguide mode. Backward-wave oscillations of various origins were suppressed by ceramic loading technology. With the electron beam voltage of 66 kV and a beam current of 11 A, a saturated peak power of 110 kW was measured at 33.9 GHz, corresponding to a saturated gain of 34.4 dB, with a -3-dB bandwidth of 1.75 GHz.

Thermal and deformation analysis of W-band gyrotron traveling wave tube amplifier (Gyro-TWTA) electron gun are carried out by using the finite element code ANSYS in the paper. Temperature distribution and thermal deformation of the cathode component at given heater power are simulated. These results are verified experimentally in an electron gun. The measured temperature distribution is in agreement with the simulation prediction. Finally, the electron trajectories with and without considering deformation are simulated by EGUN code.

In this paper, a circular TE62 mode generator is simulated and measured in order to test the quasi-optical mode converter system, which would be used in a W-band gyrotron with TE62 mode. The mode generator sequence is rectangular waveguide TE10 - coaxial waveguide TE61 - cylindrical waveguide TE61 - rotating TE61 mode - rotating TE62 mode. Simulation and calculation of the mode generator has been achieved. The optimum converting efficiency of the desired mode is approximately 90%. The output field pattern of the mode generator is presented in a temperature-sensitive liquid crystal display (LCD) sheet by the low-power microwave.

In this paper, the simulation of transverse sweeping system (TFSS) and thermal analysis of undepressed collector for a 94GHz, 30kW CW gyrotron are presented. A smooth power-density distribution profile on the collector is obtained through repeated comparison of the TFSS simulation results, which indicates the range of spent beam spread and the peak power density are about 370mm and 130W/cm(2), respectively. The sensitivities of the TFSS's parameters to the power distribution profile are also studied and analyzed. To avoid the melting of the collector, a cooling channel groove is designed and the thermal analysis with finite element software ANSYS is performed under the non-uniform heat flux. The maximum outer and inner surface temperatures are 94.3 and 102.4 degrees C with water temperature at 20 degrees C, respectively.

In this paper, an input coupler for W band gyrotron TWT (Traveling Wave Tube) has been designed. This input coupler has the ability of efficiently converts a TE10 rectangular waveguide mode into a TE01 cylindrical waveguide mode. The measured results agree well with the simulated results. With the measured -2dB bandwidth of more than 4GHz, the input coupler can be well used in the experiment of W band gyrotron traveling wave amplifier.

In this paper, the design and simulation of a high frequency structure for a W-band quasi-optical gyrotron oscillator, which operates at the TE62 mode, has been presented. According to the self-consistent nonlinear theory, the parameters of the high frequency resonator has been designed and optimized. The effects of electron beam voltage, current, velocity spread, magnetic field and electron beam guiding center on the output power and efficiency have been analyzed. The maximum output power and efficiency of the designed gyrotron oscillator is about 39.7kW and 44.1%, respectively.

A gyrotron capable of both frequency and power tuning is a promising coherent millimeter-THz wave source. A self-consistent nonlinear theory is applied to investigate the electron cyclotron interaction between electron beam and wave modes of axial nonfixed profiles in an extended W-band TE01 mode cylindrical cavity. It is revealed that tuning the magnetic field strength can excite electron cyclotron resonances on forward wave, backward wave, and even simultaneous on both waves, which makes the system operate under distinctive states, namely the gyrotron backward wave oscillation state and the gyromonotron state. In this paper, a W-band prototype gyrotron oscillator based on an extended cylindrical waveguide cavity is built, and the experiment test indicates that the system starts oscillation in a relative wide range of the operation parameters. The measured frequency spectrum reveals the system iteratively switches between the lower order instability axial modes, and it operates under nonstationary oscillation states. The experimental measurement of highest output power similar to 8 kW is consistent with the theoretical predictions. An optimized gyrotron circuit with efficiency exceeding 20% and tunable bandwidth over 10 GHz is also presented. The free oscillation behaviors revealed in this paper provide interesting guidance for developing tunable gyrotrons in millimeter-THz wave range.

The design of a high power gyroklystron amplifier operating with circular TE 011 cavity modes is presented. The amplifier will be driven by a 70-kV, 6-A electron beam with a pitch angle (v t /v z ) of 1.4 and velocity spread of 2.4% generated by a double-anode magnetron injection gun (MIG). The amplifier is predicted by particle-in-cell simulation to generated 145 kW at 94.1GHz with 35% efficiency, 38-dB saturated gain and 0.8GHz bandwidth.